1. Field of the Invention
The invention relates generally to ink jet printer technology. More particularly, the invention relates to ink jet printers which employ piezoelectric elements for ejecting ink.
2. Background of the Related Art
There are currently two major technologies used in drop-on-demand ink jet printing: thermal technology and piezoelectric technology. Most currently available ink jet printers use thermal methods to eject ink droplets out of a nozzle and onto a recording medium. In these methods, the actual ejection is initiated by heating the ink adjacent to the nozzle with a thin film resistor to create a bubble which forces a drop of ink out of the nozzle. Some recently introduced ink jet printers employ piezoelectric technology to achieve the same end of ejecting ink onto the recording medium.
Piezoelectricity refers to the deformation of a crystalline material when subjected to an electrical potential. Instead of using heat to eject the ink, these printers employ piezoelectric deformation to reduce the volume of a small ink reservoir, thereby ejecting a droplet of ink from the reservoir. In some piezoelectric ink jet print heads, a piezoelectric element is actuated so as to exert mechanical pressure on a membrane laying against the ink channel. When a very short electrical pulse is applied to the piezoelectric element, it may expand, contract, bend, or otherwse deform. The deformation of the piezoelectric element forces the ink out of the ink channel onto the recording medium. The expansion and contraction occurs at high speed and produces high pressures inside the ink reservoir, making an ink droplet eject from the nozzle and onto the recording medium.
In order to enhance printing resolution, ink jet printers often use several hundred adjacent nozzles, each having a diameter of less than 50 micrometers. The use of smaller ink chambers and finer nozzles creates a commonly recurring problem in ink jet printers. The ink channels of these printers may contain non-ink material such as air bubbles. Air can be introduced if the ink channels are run completely out of ink during use, or bubbles in the ink can become trapped near the piezoelectric actuators and nozzles over time. The presence of excess air in the channel causes the ink ejection mechanism to malfunction, thereby affecting the quality and resolution of the printed material. Such degradation in print quality can seriously undermine the effective utility of ink jet printers.
Several attempts have been made to detect the presence of air bubbles in ink channels with varying degrees of detectability. One attempt involved activating the piezoelectric element simultaneously with a simulation capacitor, and comparing the responses to the pulse activation. This technique is described in detail in U.S. Pat. No. 4,498,088 to Kanamaya. Another technique actuates the piezoelectric element with a normal ink ejection pulse, and detects a voltage overshoot which may develop across the actuated piezoelectric element. This technique is described in U.S. Pat. No. 5,500,657 to Yauchi et al. The Kanamaya and Yauchi et al. references are hereby incorporated by reference in their entireties.
The Kanamaya and Yauchi techniques require fairly complex analog actuation and detection circuits. Furthermore, they attempt to detect small perturbations in relatively large actuation signals, thus increasing the chances of erroneous evaluation of an ink channel.
The present invention provides an improvement over the prior art by simplifying the dedicated detection circuitry required for ink ejector evaluation. Advantageously, in some systems in accordance with the invention, computational hardware already present in the ink jet printer is used to perform ink ejector analysis, thereby minimizing costs associated with faulty jet detection systems.
In one embodiment of the invention, a fault detection circuit for a piezoelectric ink jet printer comprises a driver circuit coupled to at least one piezoelectrically actuated ink ejector for applying a test signal to the ejector and a pre-processing circuit for monitoring, processing, and digitizing a response of the ejector to the test signal. The fault detection circuit also includes digital signal processing means for receiving an output from the pre-processing circuit and for analyzing a frequency dependent impedance of the ink ejector. As the impedance may shift with the presence of air bubbles in the channel, faulty ink ejectors may be detected.
In another embodiment, an ink jet printer incorporating fault detection comprises a first drive circuit coupled to a plurality of ink ejectors so as to control ink ejection therefrom during normal printing operations as well as a second drive circuit periodically coupled through a resistor to a selected one of the plurality of ink ejectors. The second drive circuit is configured to apply a test signal through the resistor to the selected ink ejector. The printer also comprises a fault detection circuit having an input connected to at least one side of the resistor; wherein an electrical signal present there is detected by the fault detection circuit, and wherein characteristics of the detected electrical signal are indicative of an operational status of the selected ink ejector. It can be appreciated that in these embodiments, faulty ink ejection channels may be accurately detected using a minimum of dedicated circuitry.
Methods of detecting faulty ink ejectors are also provided. In one embodiment, an ink jet printer system has a plurality of ink jet channels (IJC), each IJC including a piezoelectric element. A method of detecting faulty IJCs includes driving the piezoelectric element with an input voltage signal; and sensing a phase difference between the input voltage signal and a resulting current through the piezoelectric element. In another embodiment, a method of detecting faulty IJCs comprises determining the impedance of the piezoelectric element at at least one frequency band. The above described methods take advantage of variations in a piezoelectric ink ejectors response to selected test signals, reducing the complexity of test driver and detection circuitry.